Patrick Pease

Aerosols over the Arabian Sea: Atmospheric transport pathways and concentrations of dust and sea salt

This paper provides an overview of dust transport pathways and concentrations over the Arabian Sea during 1995. Results indicate that the transport and input of dust to the region is complex, being affected by both temporally and spatially important processes. Highest values of dust were found off the Omani coast and in the entrance to the Gulf of Oman. Dust levels were generally lower in summer than the other seasons, although still relatively high compared to other oceanic regions. The Findlater jet, rather than acting as a source of dust from Africa, appears to block the direct transport of dust to the open Arabian Sea from desert dust source regions in the Middle East and Iran/Pakistan. Dust transport aloft, above the jet, rather than at the surface, may be more important during summer. In an opposite pattern to dust, sea salt levels were exceedingly high during the summer monsoon, presumably due to the sustained strong surface winds. The high sea salt aerosols during the summer months may be impacting on the strong aerosol reflectance and absorbance signals over the Arabian Sea that are detected by satellite each year.

Mineralogical characterization and transport pathways of dune sand using Landsat TM data, Wahiba Sand Sea, Sultanate of Oman

Sand samples and Landsat imagery were used to characterize the spatial distribution of sand mineralogy, and to evaluate potential sources and transport pathways of sediment in the Wahiba Sand Sea in the Sultanate of Oman. Landsat TM data were useful in the geomorphic interpretation of a desert, aeolian environment to distinguish the mineralogies found in the Wahiba area, and in extending the identification of sand mineralogy beyond the point-specific grid produced from field sampling. The Wahiba Sand Sea has been previously divided into upper (north and higher in elevation) and lower (south and lower in elevation) portions, based on dune morphology. Large-scale spatial distributions of mineral composition follow a similar pattern to that of the geomorphic regions within the dune field. Field and Landsat TM data suggests the upper Wahiba is comprised of well-mixed carbonate and quartz sand with some areas of significant mafic content. Surface sand in the eastern and western margins of the upper Wahiba originate from the lower Wahiba and from local wadis. Sand in the lower Wahiba was likely derived from southern sabkhas and the coastal area and is quartz-rich with isolated locations of carbonate-rich sand. We suggest that these carbonate-rich patches in the lower Wahiba were derived from reworked aeolianites (lithofied sand dunes) which underlie most of the sand sea, and not directly from the coast, as previously reported.

Composition and Sources of Sand in the Wahiba Sand Sea, Sultanate of Oman

The Wahiba Sand Sea in the Sultanate of Oman is composed of two physiographic units that can be roughly divided into northern and southern regions. The Northern Wahiba is predominantly a large megaridge system, whereas the Southern Wahiba mostly comprises linear dunes, sand sheets, and nabkha fields. Although the dunes of the two regions are of different ages, it has previously been hypothesized that their sands were derived primarily from the same source, namely coastal sands. However, mineralogical, geochemical, and grain–size data in this study suggest that the two regions have different sources. The Northern Wahiba sands have a high composition of mafic minerals and came primarily from local wadis that drain the adjacent Hajar Mountains. One of the prominent wadi sources is presently buried under sands of the Southern Wahiba, eliminating it as a current source of the Northern Wahiba. The Southern Wahiba sands have a more mineralogically mature, quartz–rich composition and are derived from separate sources, likely to be the Oman coast or adjacent sabkha plains. Previous researchers used carbonate sand, present throughout the sand sea, to infer that the coast was the major source of sands in the entire sand sea. However, geochemical evidence suggests that the carbonate grains in the sand sea originated from the two contrasting sources along with the other grains. Carbonate grains in the Northern Wahiba are derived from wadis that drain limestones in the Hajar Mountains whereas carbonate grains in the Southern Wahiba are probably reworked from underlying aeolianites and coastal sands.

Modelling alluvial fan morphology

A mathematical model which estimates the scale-independent sediment surface profile of alluvial fans has been developed. This model utilizes a diffusive sediment transport model and an unsteady, radial flow, conservation relationship. These equations are approximately solved assuming a quasi-steady-state closure with appropriate modelling assumptions for two end member fan types: (1) fans where most of the fan surface is depositionally active (denoted here as ‘homogeneous’) and (2) fans characterized by channelling and sediment sorting processes. The fundamental result for these two fan types is a dimensionless sediment profile relationship which approximates most fan surfaces. The model suggests that the overall dimensionless morphology of alluvial fans is governed more by fundamental diffusion principles in sediment deposition than by individual environmental or basin characteristics. Additionally, this work potentially can be extended to model temporal variation in fan development. Preliminary comparison with alluvial fan profiles is reasonable, indicating that this model provides useful qualitative and quantitative information relating to alluvial fan process and morphology. Copyright

Geochemistry of sediments from Quaternary sand ramps in the southeastern Mojave Desert, California

Abstract Trace element analyses of sediment from sand ramps in the Bristol Trough and Clarks Pass aeolian corridors of the Mojave Desert, California were conducted to determine their depositional history and relation to sources of sediment within sand transport corridors. Sand ramps are topographically controlled depositional systems consisting of amalgamated accumulations of aeolian, fluvial and talus deposits. These landforms contain a variety of deposits formed in different environments and are, therefore, a valuable source of paleoenvironmental information. Sediments were studied from three sand ramps, the Iron Mountain, Big Maria, and Dale Lake sand ramps; and from two sand sheets, Rice Valley and Cactus Plain. Cactus Plain is located on the eastern side of the Colorado River, in Arizona, but previous researchers have suggested that it is related to the corridors of the western (California) side. Geochemical data from units within individual sand ramps indicate that sources for each sand ramp changed through time, probably as sediment availability from different local fluvial/playa systems changed in response to climate fluctuations. Analyses also indicate that each sand ramp is composed of sediment from discrete, local sources. Sand deposits in the Bristol Trough are not integrated, and thus the corridor does not act as a coherent sand transport pathway. Comparisons of sand from Cactus Plain and sediment from the Bristol Trough and Clarks Pass corridors indicates that the Cactus Plain sand, on the east side of the Colorado River was not derived from sources on the west side of the river.

Floodplain Sedimentation During an Extreme Flood: the 1999 Flood on the Tar River, Eastern North Carolina

This study examines floodplain sedimentation following the largest flood in the 98-yr. record on the Tar River, North Carolina. Hurricane Floyd made landfall just 10 days after Hurricane Dennis in September 1999, bringing unprecedented rainfall (30-46 cm) and flooding to eastern North Carolina. A field survey of the lower 350 km of the river showed that this >500 yr. flood deposited very little overbank sediment (<1 mm) on most of the floodplain. We used suspended sediment concentrations measured on the Tar River from 1958-1967 to suggest that the seasonal timing and sequencing of flood events in 1999 are the most probable explanations for the minimal geomorphic impact of this extreme flood. The early autumn timing of the flood coincided with crops that were mature but not yet harvested, and when natural vegetation was very dense and effective at stabilizing channel banks, hillslopes, and floodplain soils. Hurricane Dennis may have exhausted the available sediment supply and transported this sediment to the Pamlico Sound before reaching flood stage, thereby reducing the sediment available to be transported and deposited by the flood that followed Hurricane Floyd.

EOLIAN DUST EROSION FROM AN AGRICULTURAL FIELD ON THE NORTH CAROLINA COASTAL PLAIN

Eolian erosion typically has not been considered a significant process on the humid southeastern coastal plain of the United States. A preliminary study of eolian erosion from an agricultural field was undertaken during the late winter of 2002 and early spring of 1999. During those times local agricultural practices leave fields bare while frontal systems produce frequent high wind events. Dust emissions were measured with two samplers; modified Wilson and Cooke passive dust traps and high-volume air samplers. Results of the study indicate that wind erosion is a significant process on agricultural fields of the North Carolina Coastal plain. Dust flux off of the field during the largest of five measured events was estimated as high as 126 kg/m with total losses of 3070 kg/ha. Atmospheric concentrations of suspended material were measured at 58,815 μgm-3. Sediment erosion was not evenly distributed across the field. Erosion was focused over soils that are better drained. Low levels of soil moisture did not eliminate erosion but instead produced pulses of sediment emission as sustained wind continually dried then activated sequential layers of the field surface. Soil moisture and topography appear to be the primary controls on spatial erosion differences and soil characteristics likely play a secondary role.

Suggestions for Low-Cost Equipment for Physical Geography II: Field Equipment

Abstract Fieldwork and laboratory experiences have always been important components of physical geography education, at universities as well as secondary schools. However, the rising cost of necessary equipment and dwindling education budgets of most universities and secondary schools have placed such experiences in crisis. This is the second of two papers that present lab- and field-based items we have designed and built for student research. The equipment is easy to construct and made from low-cost materials like PVC plumbing pipe. Photographs, construction notes, and costs have been included for each of the pieces of equipment, as well as measured schematics for the more complex items.

Drainage Ditches as Sediment Sinks on the Coastal Plain of North Carolina

This paper examines the role that slope-channel linkages and seasonal variations in vegetation play in explaining spatial and temporal variations in sediment flux through agricultural drainage ditches in eastern North Carolina. We used biannual cross-sectional surveys of drainage ditches to assess erosion/deposition during a five-year period in the headwaters of a small agricultural watershed. Although net accumulations of sediment were observed in three-fourths of the cross sections surveyed, the rate of sedimentation varied considerably from ditch to ditch and cross section to cross section. The ditches were sediment sinks during the growing season in summer and autumn when they became choked with dense vegetation growth, and more hydraulically efficient after removal of vegetation in December during annual maintenance operations. The ditches experienced erosion or modest deposition while the vegetation was dormant during the late winter/early spring. Sediment was delivered to the ditches from isolated gullies that linked the primary source of sediment, soil eroded on agricultural fields, to the channels. Except for these isolated linkages, ditches and fields are largely decoupled.

Magnetostratigraphy of cave sediments, Wyandotte Ridge, Crawford County, Indiana: towards a regional correlation

Abstract The polarity of 42 sediment samples obtained from 21 sites in Wyandotte Cave and five smaller satellite caves in Wyandotte Ridge, southern Indiana, is used as a basis for a magnetostratigraphic correlation with sediments in Mammoth Cave, Kentucky. Sediments in caves at elevations between 137 and 162 m have a normal polarity, but the fill between 168 and 171 m exhibits a polarity reversal. The reversal is interpreted to represent the most recent polarity change, suggesting that the upper level of Wyandotte cave was last active at about the same time (not less than 0.78 Ma) as the “C” level in Mammoth Cave which lies at approximately the same elevation. The local base level control in both cave systems is provided by tributaries of the Ohio River. Thus, the correlation is most likely a consequence of the contemporaneous abandonment of upper-level passages in Wyandotte and Mammoth caves during the early Pleistocene reconstruction of the Ohio River drainage. Sediments in caves between 236 and 241 m have a normal polarity. The fill in these (Little Wildcat and Galley Door) caves is probably at least 2.6 Ma old. It appears to correlate with the residuum that delimits the Upper Mitchell Plain Surface (the Pennyroyal Plateau in Kentucky) and, on the basis of our interpretation of the magnetostratigraphy, is older than the fill in the highest (“A” and “B”) levels in Mammoth Cave.